In recent years, along with popularization of mobile terminals such as mobile telephones or portable note type personal computers, importance of a power supply thereof receives widespread attention. It has been demanded for the power supplies that they are reduced in the size and the weight, as well as they have high capacity and are less deteriorated. Since a lithium ion secondary battery has a high operation voltage and high energy density, it is suitable as a battery for such mobile terminals. Further, a direct methanol fuel cell (DMFC) has also attracted attention as a power supply for mobile terminals.
In the lithium ion secondary battery, an electrode active material using LiCoO2 or manganese spinel as a main ingredient has been used for a cathode material, and a carbon material including graphite has been used for an anode material. For molding the electrode active materials as an electrode, carbon particle or a binder referred to as a conductive aid material is used being kneaded with electrode active material. Then, the current flowing path includes a path in which current flows from the surface of the active material where electrode reaction proceeds, by way of skeletons such as the conductive aid material to the outside. However, since the electrode active material is particles independent from each other, the resistance is high and there are many particles not substantially contributing to the electrode reaction.
With an aim of enhancing the safety of the lithium ion secondary battery, it has been proposed to use a solid electrolyte. However, since it is difficult to uniformly contact the active material and the solid electrolyte to each other, this increases the active material not contributing to the electrode reaction.
Further, in the direct methanol fuel cell, a noble metal such as platinum referred to as a catalyst is supported as a nano size particle on a carbon material at each of the electrodes of the fuel electrode and the air electrode. Also for such electrodes, it has been proposed to use noble metal particles as the electrode being kneaded and molded with a binder and use the same in combination with a solid polyelectrolyte. However, it can not be said that they are utilized effectively, for example, in that the nano size catalyst is buried in the electrolyte.
Technical documents concerning the prior art include, for example, JP-A No. 2004-349164 and JP-A No. 50-36935.
The electrode of a battery comprises an active material contributing to the electrode reaction, a conductive aid material to be a conduction path for the active material and a current collector for flowing current externally. However, in a case where the active material is not physically bonded to the conductive aid material or in a case where the current flowing path from the active material to the current collector is long and, accordingly, the resistance is high, since the conduction path from the active material of the battery to the outside is restricted, it is considered that the internal resistance is high and an active material not capable of contributing to the electrode reaction is generated.
Further, an conventional electrode structure involves a problem of causing peeling between the active material and the conductive aid material due to volumic change or stress along with charge/discharge. Therefore, the deterioration of current collection or deterioration of capacity due to powderization may increase. Since the internal resistance of the battery increases due to the deterioration of the current collection, it also results in a problem capable of not obtaining a satisfactory battery characteristic. It is considered that they are caused by the following phenomenon. That is, expansion/contraction of a metal contained in the carbon material makes the density between particles constituting the electrode coarser. As a result, the conduction path decreases and, further, the conduction path becomes incomplete upon repeating charge-discharge, and then a portion not contributing to charge/discharge may occur.
It may be possible to improve the safety of the battery by using a polymer type solid polymeric membrane for the electrolyte. However, the conventional electrode structure has a problem that formation of interface is difficult between the active material of the electrode and the solid electrolyte membrane, as a result, active material not substantially capable of contributing to the reaction is present.
As described above, while the structure of the electrode in the electrochemical device has a great concern with the performance thereof but there exists a common subject that the active material can not yet been utilized effectively.
The present invention has been achieved in view of the foregoing problems and it intends to provide an electrochemical device-electrode having characteristics of high capacity and low resistance. Particularly, the present invention is applied to the electrode such as an anode for use in lithium ion secondary battery, a cathode for use in alkali storage battery, an electrode for use in fuel cell or a capacitor electrode.